A central question of developmental biology is how polarity is established and maintained. The proper asymmetric distribution of cytoplasmic and membrane components between daughter cells is essential to the emergence of diversity during embryonic development. One factor that has long been mentioned as a possible determinant of polarity is an endogenous electrical field. Many cells in culture respond to applied electrical fields in a polarized manner, but conclusive evidence for a role for endogenous fields is lacking. The development of the Drosophila central nervous system requires the asymmetric division of stem cells called neuroblasts, which produce a series of ganglion mother cells (GMC). It has recently been found that polarity of the neuroblasts in culture is exceeding sensitive to applied electrical fields, with GMC forming on the anodal side. As the neuroblasts acquire their first polarity in an epithelial layer, which is electrically polarized, it is reasonable to hypothesize that the trans-epithelial potential (TEP) is an important factor in neuroblast polarity. The proposed research will critically test this hypothesis. The specific aims of the proposal are to (1) Measure the development of the TEP as the formation of the cellular blastoderm proceeds and determine the physiological basis for the TEP, (2) Determine if an electrical field is sufficient for NB polarity in vitro, (3) Develop physiological and genetic means of manipulating the TEP and determine if an electrical gradient controls NB polarity in vivo, and (4) Determine the role of cytoplasmic calcium gradients on NB polarity formation and the response to applied electrical fields. From this work, definite evidence, negative or positive, of a role of endogenous fields in development will emerge as well as information about the mechanism of field interaction with cells. Endogenous electric fields are ubiquitous in development and arise whenever an epithelial layer is damaged, so these results will have implications for understanding development as well as wound healing.